The present invention is directed to a circuit arrangement having a switching amplifier, particularly for hearing aids, for limiting a pulse-width-modulated signal that is formed by a low-frequency signal and a higher-frequency delta signal.
German Patent 36, 16 752 discloses a circuit arrangement having a switching amplifier for hearing aids. In this known switching arrangement, a pulse-width-modulated signal is formed from a low-frequency audio signal and from a higher-frequency delta signal in a pulse duration modulator. After amplification of the pulse-width-modulated signal in the switching amplifier, the audio signal is detected from the different duration of the individual pulses of the pulse-width-modulated signal. This usually occurs in hearing aids on the basis of the low-pass effect of an earphone connected to the output of the switching amplifier.
In a pulse-width-modulated signal, the amplitude of the audio signal (useful signal) is coded in the width of the individual pulses of the pulse-width-modulated signal that controls the switching amplifier. The amplitude of the useful signal coded in the pulse width can therefore not be limited in the output stage (in the switching amplifier) with standard methods or clip circuits.
The audio signal (useful signal) has therefore already been limited in amplitude in a preliminary stage and a pulse-width modulation has been generated with this previously limited audio signal. As a result thereof, however, an undesirable spectral spread of the useful signal already occurs before the pulse width modulator. This results in undesirably high back-convolution products (high distortions) in the output signal.
In order to avoid this disadvantage, the operating voltage (or the operating current) for the switching amplifier has already been limited. This, however, leads to an undesirable reduction in gain and power of the demodulated output signal.